ORP/SID 72-3
REFERENCE DATA for
RADIOFREQUENCY
EMISSION HAZARD
ANALYSIS
I
ill
111
\S»
o
UJ
CD
ENVIRONMENTAL PROTECTION AGENCY
Office of Radiation Programs
-------
REFERENCE DATA FOR RADIOFREQUENCY EMISSION
HAZARD ANALYSIS
Richard A. Tell
Electromagnetic Radiation Analysis Branch
Surveillance and Inspection Division
June 1972
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Radiation Programs
Washington, D.C. 20460
-------
FOREWORD
Since the first commercial radio station began broadcasting
in 1921, the number of radio and television broadcasting stations
in this country has increased dramatically and now exceeds 6,400.
The electromagnetic energy transmitted by these installations as
well as that associated with civilian and government microwave and
radar devices has become of concern because of possible health
effects. Furthermore, most broadcast stations are located near
large population concentrations, and in fact direct their emissions
at the population.
The evaluation of the possible health hazards to a population
in a particular location involves many variables and requires data
from a number of sources. This document illustrates the methods
employed in calculating power density and other values closely
associated with environmental radiation from radiofrequency
emitters and possible health effects. Graphs and tables of
pertinent data used in the calculations are included.
Additional information concerned with irradiation and the
biological factors associated with health effects are sought on a
continuing basis. The comments of individuals interested in this
or allied aspects of radiation protection of man and his environ-
ment are solicited.
-------
ACKNOWLEDGEMENTS
The author extends his appreciation to those individuals
who played a part in producing this report: Dr. Claude Weil and
Mr. Joseph Ali for their review and technical suggestions,
Mr. Ernest Bucci for his preparation of photographs, Mr. Donald
Hodge and Mrs. Angie High for their assistance in editing and
assembly of the report, and finally Mrs. Patricia Nash for her
excellence in patience and typing.
11
-------
CONTENTS
Foreword i
Acknowledgements ii
Introduction 1
Description of Data 2
Examples 8
Tables and Graphs 11
Miscellaneous Data 23
Glossary 25
References 28
-------
INTRODUCTION
Recent attention has been directed toward the environmental
impact of nonionizing electromagnetic radiation, particularly the
relationship between environmental exposure levels and possible
health implications. Although the current status of biological
effects research precludes any specific conclusions about what
levels and frequencies of radiofrequency energy are definitely
hazardous, certain general guidelines for exposure have been
established.
Theoretical evaluation of exposure levels in the vicinity
of various radiofrequency emitting devices must incorporate the
effects of many variables. These variables include source
parameters, geographical factors, and atmospheric effects;
propagation models utilizing this information are often complex
and are difficult to use for rapid estimation applications. The
purpose of this report is to provide, in a single source, a
collection of information which is helpful in the practical
evaluation of environmental radiofrequency exposure levels from
these emitters. For the most part, this information is in the
form of graphs and tables indicating the relationship between
selected parameters. Included is a glossary of commonly used
terms.. Two examples of mathematical solutions illustrate the
data's applicability to hazard analysis.
-------
DESCRIPTION OF DATA
TABLE 1
Table 1 provides a perspective to the radiofrequency (RF)
hazards analysis area and illustrates various radiation protection
guides in use throughout the world.
TABLE 2
Table 2 gives commonly used frequency band designations as
adapted from the Radio Regulations of the International Tele-
communications Union, Article 2, Section 11, Geneva; 1959.
Reference (1).
TABLE 3
Power and voltage ratios are conveniently expressed in
decibels (dB) . The expressions relating power and voltage ratios
and dB are:
dB = 10 log^o (Power ratio)
dB = 20 Iog10 (Voltage ratio)
Table 3 allows conversion of dB to power or voltage ratios, both
greater and less than unity.
TABLE 4
Table 4 gives the general letter designations for the
various microwave frequency bands within the range 225 MHz to
56 GHz. Reference (1).
-------
GRAPH 1
For plane electromagnetic waves in free space, the electric
field strength E is related to power density PD by the relation-
ship
PD = E_ = § _ , where
Z0 377
E is expressed in terms of volts per meter; ZQ is the intrinsic
impedance of vacuum which is 377 ohms; and PD, the power density,
is in units of watts per square meter.
If field strength is specified in volts per meter and the
ry
power density is desired in units of mW/cmz , the relation is
simply
PD (mW/cm2) =
The right-hand scale of Graph 1 provides the connection between
expressing power density in mW/cm2 and dBm/cm2. Here 0 dBm is
equal to 1 mW.
GRAPH 2
Graph 2 relates effective radiated power (ERP) , distance
from the source, and the equivalent free space power density for
several selected levels. In this case, the term free space
refers to the fact that the computed values of power density were
obtained under the assumption that no reflecting surfaces, such
as the ground, caused other than the directly radiated wave
to impinge at the calculation point. Furthermore, the medium of
-------
propagation offered no attenuation to the radiated waves. The
ERP is computed on the basis of transmitter output power, power
fed to the transmitting antenna, and antenna power gain G
according to the relation.
ERP = GPt, where
G is the power gain expressed as a factor (e.g. , a 12 dB power
gain antenna will enter as a factor of 15.85 as taken from
Table 3) and P-j- is the transmitter output power. The units of
the calculated ERP will be in the same units as used for P.J-
(watts, kW, etc.) .
From this value for ERP, the field density PD is arrived
at from
PD = ERP ? , where
4 IT Rz
R represents the distance from the source.
Another useful formula is that for field strength in volts
per meter: Field strength (V/m) = \30 ERP, where ERP is
R
expressed in watts at the desired radiation angle and R is the
distance in meters.
GRAPH 3
Graph 3 indicates the variation in field strength with
distance for a maximum power 50 kW AM broadcast station. Both
frequency of emission and ground conductivity affect the ground
level field strength from the source. Two curves are given to
-------
indicate the range in possible levels caused by variations in
these two parameters. It has been assumed that the transmitting
antenna is a single monopole radiator with an optimum height of
5/8 wavelength. These data are condensed from FCC information
(2).
GRAPH 4
The radiated field strength from a vertical monopole
radiator is a function of the current distribution on the
radiator as well as the ground conductivity at the base of the
tower and over the path of radiation to the reception point.
This graph indicates the field strength of towers of various
electrical heights for sinusoidal current distributions and high
conductivity grounds. The optimum tower height is seen to be
0.625 \. These data are condensed from the FCC Rules and
Regulations (2) .
GRAPH 5
Most television and FM broadcast stations employ trans-
mitting antennas which exhibit gain in vertical planes. This
means that the radiation field is restricted to some small
vertical angle of emission; i.e., rather than radiating at useless,
high vertical angles, the beam is flattened to propagate most
of the energy in a narrow beam which is usually aimed at the
horizon or some slightly lower angle. The radiation character-
-------
istics of the antenna are usually uniform in the horizontal
plane, i.e., it radiates equally in all azimuthal directions.
The primary power gain of such antennas is thus obtained in the
vertical plane. This contrasts with vertical radiators, such
as AM broadcast stations, in which horizontal directionality is
sometimes desired and obtained by strategically placing a number
of towers in a phased array.
Graph 5 illustrates the vertical gain pattern of a typical
medium gain UHF TV transmitting antenna. Here, the ordinate is
expressed as the relative field strength. Thus, for any parti-
cular depression angle, the field strength may be determined in
relation to whatever the main beam field strength would be at
the same distance from the tower. Ground level field strengths
may thus be easily computed if the ERP in the main beam is known.
GRAPH 6
This graph related antenna depression angle to distance
from the antenna for various antenna tower heights. Generally,
broadcasting antennas for FM and television service are
directive in the vertical plane; i.e., the antenna concentrates
the power at some specific angle with respect to the horizontal
plane. This means that the radiation intensity varies as a
function of height above ground, for a given ground distance
to the tower. The depression angle is defined as that angle
below the horizontal plane at the antenna's height defined by
-------
a line drawn from the reception point on the earth's surface to
the antenna. As the surface distance from the tower to the
observation point decreases, the depression angle increases.
This angle is used in evaluating the field strength of such an
antenna at ground level, or any other level, which is not in the
main beam of the antenna. A vertical gain pattern for the spe-
cific antenna is required in order that the appropriate power
gain at the particular depression angle of interest may be used
to compute the exposure level.
GRAPH 7
In a radar transmitter, the ratio of the average power to
the peak power is called the duty factor, or
Duty factor =
= p
P peak
Also, the duty factor is equal to the product of the pulse width
and pulse repetition frequency (PRF) . Various combinations of
these radar parameters are given. Information from reference
(1).
GRAPH 8
This graph allows estimation of the antenna power gain for
parabolic dish-type radar and microwave antennas when the dish
diameter is known. The right-hand scale yields the half -power
beam width for the antenna. Information from reference (1) .
-------
EXAMPLES
EXAMPLE 1
Given:
A UHF TV station operates with a transmitter power output
of +83 dBm into a medium gain antenna with a maximum power gain
of 14 dB atop a 700 ft. tower. Find the ground level field
strength and power density over flat terrain at a distance of
5.2 miles from the tower. Use the vertical gain antenna pattern
of Graph 5*for this problem.
1. First find the output power in kW.
+83 dBm = 80 dBm + 3 dBm = (10^) (2)mW = 2 x 105 W =
2 x 102 kW Output Power = 200 kW
dB's are additive; however, the power ratios are multipli-
cative as shown because of the logarithmic nature of the dB.
2. Next, the maximum effective radiated power is computed as
ERPmax = Pout * Gmax = 20° kw * 25'12 = 5'024 ™
The gain of 14 dB is expressed as a power ratio of 25.12.
3. Next, the depression angle is determined from Graph 6 as
1.5° for the 700 ft. tower and ground distance of
5.2 miles.
4. The field strength is now computed for the main beam of
the transmitting antenna at a distance of 5.2 miles as
-------
Field Strength
=>/30 Pt
R (meters) (5.2 miles) (1609 m/mile)
= 1.467 volts/meter
5. Finally, the effective field strength at ground level is
found by multiplying the main beam field strength as
computed in 3 above by the relative field factor found
from Graph 5 at a depression angle of 1.5°.
Ground level field strength = 1.467 V/m x 0.675 = 0.990 V/m.
This is equivalent, from Graph 1, to 2.60 x 10 ~4 mW/cm2.
EXAMPLE 2
Given:
A radar facility, utilizing a parabolic dish antenna
approximately 4 ft. in diameter, has a peak transmitter output
power of 2 MW. Assuming a PRF of 200 pulses per second and a
pulse width of 5 |j.sec. , find what target distance in the main
beam of the radar antenna is associated with an average field
O
density of 1 mW/cm , if the radar operates at 10 GHz.
1. From Graph 7 it is apparent that, for the above parameters,
a duty factor of .001 exists for the system and, consequent-
ly, the average output power to the antenna is 2 kW.
2. Now, from Graph 8, it is determined that the parabolic
dish exhibits a gain of 40 dB or a power factor of 104.
3. The average effective radiated power is now computed as
ERP = PtG = (2 x 103) (104) W = 2 x 107 W = 2 x 104 kW
-------
10
4. Finally, from Graph 2, it is seen that for an ERP of
/ 2
2 x 10^ kW, an average power density of 1 mW/cm will
occur at 0.25 mile from the source, this being in the main
beam of the transmitting antenna.
-------
11
Country and
Source
Radiation
Frequency
Maximum
Recommended
Level
Condition or Remarks
USA (USASI)
US Army.and
Air Force
Great Britain
(Post Office
Regulation)
NATO (1956)
Canada
Poland
German Soc.
Republic
U.S.S.R.
10 MHz to 100 GHz
30 MHz to 30 GHz
10 MHz to 100 GHz
300 MHz
Czech. Soc. Rep.
0.1 to 1.5 MHz
1.5 to 30 MHz
30 to 300 MHz
300 MHz
0.01 to 300 MHz
300 MHz
10 mW/cm
1 mW hr/cm
10 mW/cm2
Periods of 0.1 hr.
Averaged over any
0.1 hr. period
Continuous exposure
10 to 100 mW/cm Maximum exposure
time in minutes at
W(mW/cm2) = 6000W2
No occupancy
100 mW/cm2
10 mW/cm2
0.5 mW/cm2
1 mW hr/cm1
10 mW/cm2
10 nW/cm2
100 nW/cm2
1 mW/cm2
10 mW/cm2
20 V/m
5 amp/m
20 V/m
5 V/m
10 nW/cm2
100 nW/cm2
1 mW/cm2
10 V/m
25 nW/cm2
10 |iW/cm2
Continuous 8-hr.
exposure, average
power density
Averaged over any
0.1 hr, period
Periods of 0.1 hr.
8 hr. exposure/day
2 to 3 hr/day
15 to 20 min/day
Alternating magnetic
fields
6 hr/day
2 hr/day
15 min/day
8 hr/day
8 hr/day, CW operation
8 hr/day, pulsed (for
shorter exposures
see Figures 11 and
12)
TABLE 1. MAXIMUM RECOMMENDED LEVELS FOR HUMAN EXPOSURE
-------
12
TABLE 2
FREQUENCY BAND NOMENCLATURE
Frequency Range
Atlantic City
Frequency Subdivision
3 - 30 kHz
30 - 300 kHz
300 - 3,000 kHz
3,000 - 30,000 Khz
30 - 300 MHz
300 - 3,000 MHz
3,000 - 30,000 MHz
30 - 300 GHz
300 - 3,000 GHz
VLF
LF
MF
HF
VHF
UHF
SHF
EHF
Very-low frequency
Low frequency
Medium frequency
High frequency
Very-high frequency
Ultra-high frequency
Super-high frequency
Extremely-high frequency
-------
13
VOLTAGE
RATIO
I.POOO
.9988
.9977
.9964
.9954
.9943
.9931
.9920
.9908
.9897
.9886
.9772
.9661
.9550
.9441
.9333
.9226
.9120
.9016
.8913
.8810
.8710
.6610
.8511
.6414
.8318
.8222
.6128
.6035
.7943
.7852
.7762
.7674
.7586
.7499
.7413
.7328
.7244
.7141
.7079
.6998.
.6918
.6839
. .6761
.6683
.6607
.6531
.6457
.6383
.6310
.6237
.6166
.6095
.6026
.5957
.5888
.5621
.5754
.5689
.5623
.5559
.5495
.5433
.5370
.5309
.5248
.5188
POWER
RATIO
1.0000
.9777
.9954
.9931
.9908
.9886
.9863
.9840
.9817
.9795
.9772
.9550
.9333
.9120
.8913
.8710
.8511
.8318
.8128
.7943
.7762
.7566
.7413
.7244
.7079
.6918
.6761
.6607
.6457
.6310
.6166
.6026 '
.5668
.5754
.5623
.5495
.5370
.5248
.5129
.5012
.4898
.4786
.4677
.4571
.4467
.4365
.4266
.4169
.4074
.3961
.3890
.3602
.3715
.3631
.3548
.3467
.3388
.3311
.3236
.3162
.SOW
.3020
.2951
.2864
.2818
.2754
.2672
dB
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.06
0.09
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.6
1.9
2.0
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3.0
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
4.0
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.7
5.0
5.1
5.2
5.3
5.4
5.S
5.6
5.7
VOLTAGE
RATIO
.0000
.0012
.0023
.0035
.0046
.0058
.0069
.0081
.0093
.0104
.012
.023
.035
.047
.059
.072
.064
.096
.109
.122
.135
.148
.161
.175
.189
.202
.216
.230
.245
.259
.274
.268
.303
.318
.334
.349
.365
.360
.396
.413
.429
.445
.462
.479
.496
.514
.531
.549
567
585
.603
622
641
660
679
698
718
738
758
778
7?9
820
841
862
884
.905
928
POWER
RATIO
.0000
.0023
.0046
.0069
.0093
.0116
.0139
.0162
.0186
.0209
.023
.047
.072
.096
.122
.148
.175
.202
.230
.259
.268
.318
.349
.380
.413
.445
.479
.514
.549
:585
.622
.660
.698
.738
.778
.820
.862
.905
.950
.995
2.042
2.089
2.138
2.188
2.239
2.291
2.344
2.399
2.455
2.512
2.570
2.630
2.692
2.754
2.818
2.864
2.951
3.020
3.090
3.162
3.236
3.311
3.388
3.467
3.548
3.631
3.715
VOLTAGE
RATIO
.5129
.5070
.5012
.4955
.4898
.4842
.4786
.4732
.4677
.4624
.4571
.4519
.4467
.4416
.4365
.4315
.4266
.4217
.4169
.4121
.4074
.4027
.3981
.3936
.3890
.3846
.3802
.3758
.3715
.3673
.3631
.3589
.3548
.3506
.3467
.3426
.3388
.3350
.3311
.3273
.3236
.3199
.3162
.2985
.2818
.2661
.2512
.2371
.2239
.2113
.1995
.1884
.1778
.1585
.1413
.1259
.1122
.1000
.03162
.01 . .
.003162
.001
.0003162
.0001
.00003162
10-'
POWER
RATIO
.2630
.2570
.2512
.2455
.2399
.2344
.2291
.2239
.2188
.2138
.2069
.2042
.1995
.1950
.1905
.1862
.1820
.1778
.1738
.1698
.1660
.1622
.1565
.1549
.1514
.1479
.1445
.1413
.1360
.1349
.1318
.1268
.1259
.1230
.1202
.1175
.1148
.1122
.1096
.1072
.1047
.1023
.1000
.08913
.07943
.07079
.06310
.05623
.05012
.04467
.03981
.03548
.03162
.02512
.01995
.01585
.01259
.01000
.00100
.00010
.00001
io-«
io-»
10"
10"
io-"
dB
5.8
5.9
.0
.1
.2
.3
.4
.5
6.6
6.7
6.8
6.9
7.0
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
7.9
8.0
8.1
8.2
8.3
8.4
8.5
8.6
8.7
8.6
8.9
9.0
9.1
9.2
9.3
9.4
9.5
9.6
9.7
9.8
9.9
10.0
10.5
11.0
11.5
12.0
12.5
13.0
13.5
14.0
14.5
15.0
16.0
17.0
18.0
19.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
90.0
00.0
VOLTAG
RATIO
1.950
1.972
1.995
2.018
2.042
2.065
2.089
2.113
2.138
2.163
2.168
2.213
2.239
2.265
2.291
2.317
2.344
2.371
2.399
2.427
2.455
2.483
2.512
2.541
2.570
2.600
2.630
2.661
2.692
2.723
2.754
2.786
2.818
2.851
2.884
2.917
2.951
2.985
3.020
. 3.055
3.090
3.126
3.162
3.350
3.548
3.758
3.981
4.217
4.467
4.732
5.012
5.309
5.623
6.310
7.079
7.943
8.913
10.000
31.620
100.00
316.20
1,000.00
3,162.00
0,000.00
1,620.00
IO1
POWER
RATIO
3.802
3.890
3.931
4.074
4.169
4.266
4.365
4.467
4.571
4.677
4.786
4.898
5.012
5.129
5.248
5.370
5.495
5.623
5.754
5.888
6.026
6.166
6.310
6.457
6.607
6.761
6.918
7.079
7.244
7.413
7.586
7.762
7.943
8.128
8.318
8.511
8.710
8.913
9.120
9.333
9.550
9.772
10.000
11.22
12.59
14.13
15.85
17.78
19.95
22.39
25.12
28.18
31.62
39.81
50.12
63.10
79.43
100.00
1,000.00
0,000.00
10*
I0«
10'
10«
10«
10"
TABLE 3. dB CONVERSION CHART
REPRODUCED BY PERMISSION OF PACIFIC MEASUREMENTS INCORPORATED,
PALO ALTO, CALIFORNIA
-------
14
TABLE 4
MICROWAVE BAND DESIGNATIONS
Letter Designation of Band
P
L
S
C
X
J
K
Q
V
Frequency Range (GHz)
.225 - .390
.390 - 1.55
1.55 - 3.90
3.90 - 6.20
6.20 - 10.90
10.90 - 17.25
17.25 - 33.00
33.00 - 46.00
46.00 - 56.00
-------
FIELD STRENGTH AND POWER DENSITY IN FREE SPACE
10" 10'
POWER DENSITY , ( mW/em2 )
GRAPH 1. FIELD STRENGTH AND POWER DENSITY IN FREE SPACE
-------
16
2 3 4 567891
0-01
0.1 1.0 10.0
MILES FROM SOURCE
100.
GRAPH 2. DISTANCE REQUIRED TO ESTABLISH VARIOUS POWER
DENSITIES AS A FUNCTION OF ERP
-------
17
EARTH DIELECTRIC CONSTANT = 15
.00003
1 10
DISTANCE FROM ANTENNA (MILES)
100 200
GRAPH 3. GROUND WAVE FIELD STRENGTH FOR 50 kW
AM BROADCAST STATION
-------
18
270
_ 260
1 250
00
oS
| 240
1 230
LU
=! 220
1 210
0
< 200
S| 190
LU
I 18°
3 "0
U_
160
150
140
130
0.0 0.1 0.2 0.3 0.4 0.5
ANTENNA HEIGHT IN WAVELENGTH
0.6
0.7
GRAPH 4. EFFECTIVE FIELD AT ONE MILE FOR SINGLE
OMNIDIRECTIONAL MONOPOLE ANTENNA
-------
Medium Gain UHF Antenna Vertical Pattern
19
0 "•
0.5" ELECTRICAL
BEAM TILT
MAJOR LOBE
POWER GAIN-24.0
HOR.GAIN-20.3
5 4 3 Z 101 23436789
AB°VE DEGREES FROM HORIZONTAL PLANE 8EL°*
GRAPH 5. MEDIUM GAIN UHF ANTENNA VERTICAL PATTERN
COURTESY RCA CORPORATION, COMMUNICATIONS
SYSTEMS DIVISION, CAMDEN, NEW JERSEY
-------
20
100°
O.I
0.1
MILES
GRAPH 6. DEPRESSION ANGLE VERSUS DISTANCE
FOR VARIOUS TOWER HEIGHTS
-------
21
PEAK POWER (kW)
1000
10
1000 500 100 50 10 5
PULSE REPETITION FREQUENCY (HZ)
GRAPH 7. RELATIONSHIP OF VARIOUS RADAR PARAMETERS
-------
22
60
POWER GAIN AND ANTENNA SIZE
.2
.5
o
"H
5 I
10
20
*
50
100
A
A
PARABOLIC REFLECTOR DIAMETER (ft)
GRAPH 8. PARABOLIC ANTENNA POWER GAIN AND SIZE
-------
23
MISCELLANEOUS DATA
Basal Metabolic Rates for humans expressed on the basis of
body surface area: (Reference 3)
20 year old male 4.62 mW/cm2
20 year old female 4.10 mW/cm2
P
30 year old male 4.37 mW/cm
r\
30 year old female 4.06 mW/cm
o
60 year old male 4.13 mW/cm
60 year old female 3.89 mW/cm2
Body weight, height, and surface area for 20-24 year old
males. Reference 4.
Body weight 71.8 kg
Height 174.5 cm
2
Surface area 1.83 m
FCC requires minimum field strength at one mile for 1 kW
power (Class I AM broadcast stations): 0.225 volts/meter
(Reference 2)
Grade A television reception in UHF band - minimum field
strength: 0.005 volts/meter (Reference 2)
-------
24
Formula for calculating the distance to the far field
from an antenna:
2
Far-field distance = =^— where
X
D = maximum dimension of antenna
X = wavelength of frequency in same units as D
The far field is arbitrarily defined as that point at which the
impinging electromagnetic waves fronts exhibit no more than
22.5 degrees phase difference, i.e., they are essentially plane
waves. It is also defined as that point at which the detected
field intensity varies strictly as the inverse square of the
distance. Thus, the beginning of the far field is not a
precise distance from the transmitting antenna. The above
equation computes this approximate distance.
-------
25
GLOSSARY OF COMMON TERMS
Average Power - the time average effective power; i.e., that
power which if converted to heat would produce the same
amount of heat as some greater peak-pulsed power.
Beam Width - usually assumed to be that angle which defines the
extent of beam divergence for an antenna, at which the
radiated intensity is one-half of, or 3 dB below, the
on-axis maximum radiated intensity, for a fixed distance
from the antenna.
CW - strictly, continuous wave emission in which the radiated
power is nonvarying in time. In practice, all signals
which are not pulsed with very short pulse widths, i.e.,
radars.
dB - a ratio measure. For relationship of dB to voltage and
power ratios, see text.
dBk - a measure of power ratio, referenced to 1 kW.
dBm - a measure of power ratio, referenced to 1 mW.
dB(j.V - a measure of voltage ratio, referenced to 1 M.V.
Depression Angle - that angle below the horizontal plane at
the antenna's height defined by a line drawn from the
reception point on the earth's surface to the antenna.
Directive Array (DA) - any form of a system of radiating
elements which when operated together, give a directional
characteristic to the emitted wave. For example, some
AM broadcast stations use more than one monopole to create
a directive property to their signal, rather than radiating
equally in all directions about the antenna.
Duty Factor - in a radar transmitter, the ratio of average power
to peak pulse power. Also, the product of the pulse
repetition frequency and the pulse width.
• »
ERP - effective radiated power equal to the product of trans-
mitter output power and antenna power gain.
-------
26
Field Strength - a measure of radiation intensity In units of
volts per meter. Normally used at lower frequencies, i.e.,
below 1,000 MHz.
Free Space - a space devoid of reflecting and attenuating prop-
erties and objects.
Gain, Antenna Power - a measure of the ability of an antenna to
enhance radiation intensity in a particular direction with
respect to an isotropic, omnidirectional radiator. Usually
specified in dB.
Ground Conductivity - a measure of the soil's electrical conduc-
tive property, and therefore, its ability to reflect radio
signals. The higher the conductivity the more reflective
it is. Usually specified in mmhos/meter.
Input Power - usually refers to the final circuit electrical
input power of a transmitter and is computed generally as
the product of final amplifier stage current and voltage.
This is always higher than the actual output power,
according to the amplifier's efficiency.
Intrinsic Impedance - a measure of the wave interacting property
of a medium. Also called characteristic impedance. For
free space, 377 ohms.
mmho/meter - a measure of conductivity for a unit path length
through a given material. A mho is equal to a reciprocal
ohm.
Monopole - a single vertical type of radiating element, usually
driven with respect to a series of buried radial conductors
forming a ground plane antenna.
Peak Power - the maximum power in a single short duration pulse
in any pulsed RF source.
PRF - pulse repetition frequency; i.e., the number of pulses
occurring during one second.
Pulse Width - the time duration of a pulse usually measured in
units of microseconds for radars.
-------
27
Relative Absorption Cross-Section - a measure of the absorption
properties for an absorbing object; a dimensionless number
determined as the ratio of the actual effective area for
power absorption to the geometrical cross-sectional area;
may be greater or less than unity.
Sector Scan - a scan by a radar antenna which Includes a
fractional angular part of 360 degrees. The antenna
oscillates back and forth over the particular sector of
interest rather than revolving continuously.
Skin Depth - that distance below the surface of a conductor
where the current density has diminished to 1/e of its
value at the surface.
Vertical Pattern - normally the gain pattern of an antenna In
the vertical plane. Most TV transmitting antennae employ
some degree of vertical gain while maintaining omni-
directional characteristics in the horizontal plane.
-------
28
REFERENCES
1. Reference Data for Radio Engineers. Howard W. Sams and Co.,
Inc. A subsidiary of International Telephone and Telegraph
Corp., ITT. 1969.
2. FCC Rules and Regulations, Volume 3, Washington, B.C.
3. Handbook of Biological Data. October 1956. Wright Air
Development Center, Air Research, and Development Command,
United States Air Force, Wright-Patterson Air Force Base,
Ohio.
4. Biology Data Book. Federation of American Societies for
Experimental Biology, Washington, D.C. 1964.
fi U. S. GOVERNMENT PRINTING OFFICE : 1972—ll81t-V37 (336)
------- |